1. Field of the Invention
The invention relates to diagnostic systems for semiconductor substrates, and more particularly, to systems for measuring substrate curvature and stress during rapid thermal processing (RTP) cycles.
2. State of The Art
During the manufacture of semiconductor wafers, it is conventional practice to form one or more thin film layers on the surface of a single semiconductor wafer to serve as a substrate for integrated circuits. The surface films can comprise, for example, silicon dioxide, AlSi, Ti, TiN, PECVD oxide, PECVD oxynitride, doped glasses, silicides, and so forth. The thickness of such films typically ranges from about 500 to about 12,000 Angstroms.
When manufacturing semiconductor integrated circuity, it is important to have minimal stresses in surface films on the underlying substrates. The surface stresses can cause, for instance, silicide lifting, the formation of voids and cracks, and other conditions that adversely affect integrated circuit semiconductor devices that are fabricated on the wafers. The surface stresses are especially problematical in large-scale integration and very large-scale circuit integrations.
Stress in surface films on semiconductor wafers can be characterized as either of the compressive or tensile type. Both stress types can cause slight curvatures in the surface of a semiconductor wafer--that is, the stresses can cause the surface of a semiconductor wafer to deviate from exact planarity. Typically, the extent of deviation is quantified in terms of the surface's radius of curvature. In practice, the radius of curvature of a semiconductor wafer is often measured in kilometers--or even hundreds of kilometers--while the diameter of a semiconductor wafer is measured in millimeters, and the depths of the surface layer films are measured in angstroms.
Detection of stresses in surface films on semiconductor wafers is important in semiconductor fabrication operations. The measurements can be used, for example, to identify wafers that are likely to provide low yields. Also, the measurements can be used to identify wafers that are likely to produce failure-prone semiconductor devices.
As mentioned above, stresses in surface films are not measured directly but, instead, are inferred from measurements of the radius of curvature of the surface of interest. In mathematical terms, surface film stresses are often expressed by a function that includes Young's modulus for the silicon substrate portion of the wafer, the Poisson ratio for the substrate, the thickness of the substrate, the film thickness, and the radius of curvature of the wafer due to surface film stress. As a matter of convention, negative values of a radius of curvature indicate compressive stress and positive values indicate tensile stress.
FIG. 1 shows an example of a known system for making laboratory measurements of surface curvatures of semiconductor wafers. In the system, a beam of laser light is directed onto the surface of a semiconductor wafer and the reflected light is projected onto a screen. If the wafer surface has a radius of curvature, the location at which the reflected light strikes the screen will change as the wafer is moved perpendicularly to the beam. By measuring both the distance that a wafer is moved and the resulting distance that a beam of reflected light moves across the screen, the wafer's radius of curvature can be determined.
In mathematical terms, the radius of curvature (R) of a wafer can be related to measurements provided by the system of FIG. 1 as follows: EQU R=2L(.delta.x/.delta.d) (1)
where .delta.x is the distance of translation of the wafer, .delta.d is the resulting translation of the spot formed by the reflected beam on the screen, and L is the distance traveled by the reflected beam. In the system shown, the beam travel distance is about ten meters (i.e., L=10 meters). These systems--often referred to as optically levered systems--are further described in Thermal Stresses and Cracking Resistance of Dielectric Films on Si Substrates, A. K. Sinha et. al., Journal of Applied Physics vol. 49, pp. 2423-2426, 1978. In practice, calibration of such systems is difficult and normally requires two or more standard reference surfaces.
The calibration problem is addressed in U.S. Pat. Nos. 5,270,560, 5,233,201, 5,227,641, and 5,118,955 all to Cheng and incorporated herein by reference. The Cheng patents provide self-calibrating systems which track the incidence of the measuring light beam on the sensing device by using differential signals generated by bifurcated sensors. The differential signals are used to control a relative translation between the sensor and the substrate, which translation serves to keep the system in proper alignment.
Semiconductor stress analysis may be a dynamic process, in which a substrate is subjected to various stress changing conditions requiring close monitoring and control. Radiant heat systems for cycling silicon wafers and elevated temperature gas processing are common in the industry, and various methods are utilized to improve temperature uniformity as described in U.S. Pat. No. 3,836,752 issued to Anderson and U.S. Pat. No. 4,680,451 issued to Gat. It is also well known to provide the combination of laser scanning on a single diameter scan line for the development of surface curvature measurements during temperature cycling of the wafer.
However, there is a need for more accurate diagnosis of substrate behavior in response to changing stress inducing conditions, such as heat changes during rapid thermal processing (RTP). RTP cycles, which involve deposition of material coatings on the substrate under various extreme heating conditions, induce structural stresses whose measurement and diagnosis are critical for effecting quality control, behavior prediction and overall improvements in semiconductor manufacturing processes. Improved, in situ measurement techniques during rapid thermal processing would therefore significantly advance the state of the art.